Cancer treatment has evolved over time to become more targeted and less toxic to the patient. Traditional chemotherapy often has a high level of systemic toxicity. Targeted therapy uses small molecules or biologics (e.g., therapeutic antibodies) to inhibit the activity of a selected cellular protein involved in cancer development, and causes much less side effect than traditional chemotherapy. Immunotherapies such as those targeting immune checkpoints (e.g., PD-1 and PD-L1) and those involving chimeric antigen receptor T (CAR-T) cells aim to bolster the patient's own anti-cancer immune defense, and have emerged as a promising new treatment paradigm.
One of the cellular proteins that have been targeted in cancer therapy is Bruton tyrosine kinase (BTK). BTK is a member of the Tec family of protein tyrosine kinases. BTK has domains with pleckstrin homology (PH), Tec homology (TH), Src homology 3 (SH3), Src homology 2 (SH2), and tyrosine kinase or Src homology 1(TK or SH1) (Akinleye et al., “Ibrutinib and novel BTK inhibitors in clinical development,” Journal of Hematology & Oncology, 2013, 6:59). Proper expression of the BTK gene in different lymphoid regions plays a key role in normal B-cell development. BTK is also involved in signal transduction pathways for B cell activation and survival (Kurosaki, “Molecular mechanisms in B cell antigen receptor signaling,” Curr OP Imm, 1997, 9(3):309-18).
BTK functions downstream of multiple receptors, including B-Cell Receptor (BCR), receptors for growth factors and chemokines, and innate immune receptors. BTK initiates a broad range of cellular processes, such as cell proliferation, survival, differentiation, motility, adhesion, angiogenesis, cytokine production, and antigen presentation, and plays an important role in hematological malignancies and immune disorders. In a mouse model for chronic lymphocytic leukemia (CLL), BTK expression levels were shown to set the threshold for malignant transformation; BTK overexpression accelerated leukemia and increased mortality (Kil et al., “Bruton's tyrosine kinase mediated signaling enhances leukemogenesis in a mouse model for chronic lymphocytic leukemia,” Am J Blood Res, 2013, 3(1):71-83).
Ibrutinib (also known commercially as IMBRUVICA®) was the first BTK inhibitor approved by the United States Food and Drug Administration for treating mantle cell lymphoma (MCL), chronic lymphocytic leukemia (CLL), and Waldenström's macroglobulinemia (WM). In general, however, the selectivity of known BTK inhibitors is not ideal—they inhibit not only BTK, but also various other kinases (such as ETK, EGF, BLK, FGR, HCK, YES, BRK and JAK3, etc.). Known BTK inhibitors also produce a variety of derivatives. These characteristics of known BTK inhibitors lead to a decrease in therapeutic efficacy and an increase in side effects. The pharmacokinetics of known BTK inhibitors also needs to be improved. Indeed, significant variations in bioavailability of ibrutinib have been observed clinically among patients (Marostica et al., “Population pharmacokinetic model of ibrutinib, a Bruton tyrosine kinase inhibitor, in patients with B cell malignancies,” Cancer Chemother Pharmacol, 2015, 75:111-121).